As a method for sensing a trace substance, a sensing instrument using a quartz resonator has been known. This sensing instrument has a quartz sensor in which an adsorption layer for adsorbing a substance to be sensed is formed on a front surface of the quartz resonator, and measures the presence/absence or concentration of the substance to be sensed by utilizing the fact that when the quartz resonator, more particularly, the adsorption layer adsorbs the substance to be sensed, its natural frequency changes according to an adsorption amount of the substance to be sensed, and this sensing instrument is advantageous in that it is applicable to a wide range and has a simple structure as an instrument, and moreover, is capable of measuring even an extremely minute amount of substance because of its high sensitivity. Therefore, in the analysis of a disease marker substance contained in blood, urine, and the like, it has conventionally been expected that a method using the quartz sensor will be an effective method replacing a conventional method.
The present inventor has been studying the possibility of applying a quartz sensor to, for example, dioxin and PCB which are environmental pollutants, a disease marker in blood, or the like, and this method, if achieving high-precision measurement of a target substance, would be innovative. The reason is because a method using a gas-chromatography mass spectrometer and an ELISA method (enzyme-linked immunosorbent assay method), which are currently known as methods of measuring, for example, dioxin, have the following problems. The former requires an extremely high instrument cost and thus a considerably high analysis cost and takes a long period of time for analysis, and the latter is low in analysis precision, though requiring less instrument cost and analysis cost and taking a shorter period of time for analysis compared with the gas-chromatography mass spectrometer.
Incidentally, a sensing instrument using a quartz sensor, if not requiring very high measurement precision, is not difficult to manufacture, but if requiring high measurement precision, it is practically difficult to manufacture. The reason is that it is very difficult to detect a minute frequency change accurately and in a short time, and besides the adsorption of a substance to be sensed, there are many disturbance factors for frequency fluctuation.
Under such circumstances, the present applicant has developed an art in which a frequency signal of an oscillator circuit is converted to a digital signal, a sinusoidal signal specified by the digital signal is subjected to quadrature detection, a rotation vector rotating at a velocity according to a frequency difference between the sinusoidal signal and a sinusoidal signal used for the detection is prepared, and the velocity of this rotation vector is monitored, whereby making it possible to detect a frequency change with extremely high precision (patent document 1). However, the higher the sensitivity of the frequency change detection, the greater an influence that the disturbance gives to a measurement error.
In the use of the above-described rotation vector method, when pure water is first supplied into a sensing sensor including a quartz resonator, followed by the supply of a sample solution containing an antigen as a substance to be sensed, an antibody layer which is an adsorption layer provided on a quartz resonator causes an antigen-antibody reaction with the antigen to capture the antigen, so that a frequency lowers as shown in FIG. 11 and FIG. 12 described in later-described experimental examples. However, it is observed that the frequency tends to gradually increase during the measurement, which makes it difficult to detect an end point of the frequency after the supply of the sample solution, and especially when the antigen concentration is low and thus the frequency change is small, there is a problem of difficulty in detecting the end point of the frequency.
As a result of a pursuit of a cause of the above, the present inventor has come to the conclusion that this is due to self-heating of a quartz piece as is inferred from the later-described experimental examples. Specifically, when the quartz resonator is oscillated in the liquid, an equivalent series resistance of the quartz piece becomes as large as, for example, 150 ohm, so that a driving current flows in the quartz piece, which results in its self-heating. Here, FIG. 14 is a frequency-temperature characteristic chart of the quartz resonator, and it is thought that due to the self-heating of the quartz resonator, the frequency tries to move, for example, from point a to point b along a characteristic curve which is a cubic curve, resulting in a change in the frequency. The phenomenon that the frequency gradually increases is consistent with that the temperature slides in a downstream-side portion of the cubic curve in FIG. 14, and the frequency as a result increases along this curve.
Therefore, even if a high-precision method is adopted for the frequency, there is a problem that its advantage cannot be fully utilized.
Patent document 1                Japanese Patent Application Laid-open No. 2006-258787        